I’ve hit 271 mph at the speed trap in both the Centenario and Aventador SV, touching 274 coming out of the second tunnel in the SV. Both were tuned more to handle at speed rather than hit absolute top speed, so somebody could probably get a little bit more speed than I did.
I definitely agree about the GT3 RS PO being pretty high up. I’ve done 276.79 at the trap and 284 out of the tunnel, again in a probably less than ideal tune for top speed.
Pretty sure my game glitched… In the loading menu where it shows personal bests/accomplishments it says my top speed is 300 mph O.o I know that isn’t right lol. If it is, I don’t remember what car I used haha. To my knowledge, it’s the Jag D-type that is the fastest. Thank god they nerfed the Dart. My 458 could beat them most times (in H3) but every time if I raced dirty 
I wish they would bring the car-meet back and give us an actual drag strip that’s longer than the ones already in the game. Derwent isn’t bad but it’s only there 7 days every month.
No one on here mentions the mosler mt900s, with gear ratio tuning and aero tuning, it can hit 272 mph and weave around cars easy, so if you want a perfect high-speed maneuvering car, go for it, i’ve found that it’s very good in drag because it reaches it’s top speed in less than 20 seconds, it feels underpowered because of how fast it gets there.
My Agera RS goes 275, sometimes 276 but you need to be going downhill
The Ferrari 599XX Evo tuned gets up to 520 kmh/ 323mph. Get it. 
My point isn’t that the mosler is the “fastest”, it’s that the mosler can hold its top speed for longer, has better maneuverability at high speeds, and can get up to speed quickly, i’ve timed the time it takes to get to 200, it takes about 10 seconds, and you may want to tune your agera, I’ve gotten mine to go 282 coming from the tunnel side of the highway